The present invention relates to a range-finding or distance measuring apparatus for measuring a distance to an object of concern by emitting a light pulse to the object, receiving an echo light pulse reflected back from the object, and determining the distance to the object on the basis of a time elapsed from the emission of the light pulse to the reception thereof.
As the range-finding system or distance measuring apparatus of the type mentioned above, there is known an apparatus in which a laser pulse beam of high intensity is employed, as disclosed, for example, in Japanese Patent Publication No. 1632/1971. FIG. 9 shows the structure of this known distance measuring apparatus. Referring to the figure, a counter 9 starts to count clock pulses generated by a clock pulse generator 2 simultaneously with the emission of a light pulse from a pulsed laser 1. A photo-detection circuit 3 receives an echo light pulse reflected back from an object of concern or target 10 illuminated with the emitted light pulse and outputs an electric pulse signal after amplification. A pulse discriminator 8 compares the electric pulse signal output from the photo-detection circuit 3 with a predetermined threshold level to thereby separate discriminatively from spurious signal components the electric pulse component which corresponds to the echo light pulse. In response to the pulse output from the pulse discriminator 8, the count value of the counter 9 is read out and processed by a processing unit 7A to determine a distance between the range-finding apparatus and the object in accordance with the following equation (1): EQU Distance=(Count Value.times.Clock Pulse Period.times.Velocity of Light)/2(1)
In other words, the distance is determined by dividing by two a product resulting from multiplication of a time intervening between the emission of the light pulse and the reception of the echo pulse signal with the velocity of light, wherein the time mentioned above is determined on the basis of the count value of the counter 9.
As can be understood from the equation (1), resolution of the distance measurement by the known distance measuring apparatus of the type described above depends on the period or frequency of the clock pulse. By way of example, when the frequency of the clock pulse signal generated by the clock pulse generator 2 is selected to be 30 MHz which represents approximately an upper limit of the frequency to which a typical general-purpose digital IC can respond, resolution of the distance measurement is on the order of 5 m. In order to improve the resolution of the distance measurement, it is necessary to increase the frequency of the clock pulse signal. For example, if the resolution on the order of 50 cm is to be realized, frequency of the clock pulse signal generated by the clock generator 2 will have to be 300 MHz. In that case, the components constituting the distance measuring apparatus such as the counter 9, the clock pulse generator 2 and others have to be implemented by using those elements which are capable of responding at ultra-high speed or pulse repetition rate. To say in another way, the conventional general-purpose digital IC can no more be used to this end, but specific and very expensive components must be resorted to, which in turn means that the distance measuring apparatus becomes very expensive. An attempt for further enhancing the resolution must be started from development of such elements or constituents themselves which are capable of operating at a ultra-high speed.
In conjunction with the known distance measuring apparatus such as described above, it is further noted that the echo light pulse reflected back from the object 10 can be detected to thereby generate a pulse for stopping the pulse counting of the counter 9 without fail by making use of a high threshold level in the pulse discriminator 8 only on the conditions that the target object 10 is located relatively in the close vicinity of the distance measuring apparatus, that the intensity of the echo light pulse is sufficiently high and that a satisfactorily good S/N ratio can be ensured for the photo-detection circuit 3. However, when the object of concern is at a relatively remote position or when the reflection coefficient of the object is so low as to make feeble the intensity of the echo light pulse, the threshold level of the pulse discriminator 8 can not be set high. In that case, it may eventually occur that the distance will erroneously be determined due to noise components contained in the output signal of the photo-detection circuit 3 before the echo light pulse arrives at the distance measuring apparatus. Thus, in order to prevent erroneous determination due to noise, the threshold level of the pulse discriminator 8 has to be set high. In that case, however, there arises a possibility that the intrinsic echo light pulse can not be detected either for the reasons mentioned above. Such being the circumstances, limitation is undesirably imposed on the range of distance which can be measured with the known distance measuring apparatus, giving rise to a serious problem.
In conjunction with the known distance measuring apparatus, it is further noted that comparison of the output signal of the photo-detection circuit 3 with the threshold level is inevitably accompanied with a problem that the timing for generation of the pulse to stop counting changes as the threshold level varies, ultimately involving error in the distance measurement.
The above problem will be elucidated in more concrete by reference to FIG. 10. In general, the light pulse emitted by the pulsed laser 1 can not assume a rectangular waveform but a rather rounded waveform which can be approximated by a Gaussian curve or a raised cosine curve under the influence of characteristics of a laser drive circuit, relaxation time and other factors. Refer to FIG. 10 at (a). Consequently, the signal output from the photo-detection circuit 3 in response to the echo light pulse reflected back from the object assumes a waveform which is temporally broadened relative to the laser pulse waveform because of inevitable band limitation encountered in the photo-detection circuit 3, as will be seen in FIG. 10 at (b). Such temporally extended waveform is compared with a preset threshold level V by the pulse discriminator 8. In that case, the timing at which the output pulse is generated by the pulse discriminator 8 becomes different in dependence on the amplitude of the signal output by the photo-detection circuit 3. By way of example, referring to FIG. 10 at (c), there are illustrated a pulse waveform having a high amplitude by a solid line together with a pulse waveform of a low amplitude by a broken line. As will be understood from comparison of these waveforms, when the amplitude is low, the timing at which the output pulse is generated by the pulse discriminator 8 is accompanied with a time lag or delay when compared with that of the high amplitude, which in turn means that the distance will be measured longer than the actual distance. Accordingly, it is apparent that an error is also involved in the measured distance in dependence on the different reflection coefficients of the objects.
It is further observed that the intensity of light undergoes attenuation as a function of the second power of the distance. Consequently, magnitude of the error involved in the distance measurement becomes different in dependence on differences in the distance to the objects to be detected, whereby linearity of the distance measurement is unwantedly impaired.